Acute myelogenous leukemia with t(6;9)(p23;q34) and
marrow basophilia: an overview.

Abstract:

Acute myelogenous leukemia (AML) with chromosomal translocation
(6;9)(p23;q34) is a rare disease with poor prognosis and distinct
clinical and morphologic features. t(6;9) results in a chimeric fusion
gene between DEK (6p23) and CAN/NUP214 (9q34). FLT3-ITD mutation is one
of the most frequent mutations in AML and correlates with poor clinical
outcome. Prevalence of FLT3-ITD is as high as 70% among patients with
t(6;9) AML, and patients with t(6;9) AML and FLT3-ITD mutations usually
have higher white blood cell counts, higher bone marrow blasts, and
significantly lower rates of complete remission. t(6;9) is most commonly
associated with AML-FAB-M2 and is considered by some researchers to be a
separate disease entity because of its distinct clinical and morphologic
features and poor prognostic implication. Distinct morphologic features
of this entity include marrow basophilia and myelodysplasia, and
immunophenotypically, the blast cells are positive for CD9, CD13, CD33,
and HLA-DR; are usually positive for CD45 and CD38; and may be positive
for CD15, CD34, and terminal deoxynucleotidyl transferase.

Acute myelogenous leukemia (AML) with translocation involving
breaks at band 23 of the short arm of chromosome 6 and band 34 of the
long arm of chromosome 9, t(6;9)(p23;q34), is a rare disease and
constitutes 0.5% to 4% of all AML cases. (1) This translocation results
in the formation of a chimeric fusion gene (DEK-NUP214)on the derivative
chromosome 6, der(6), and was first identified in AML by Rowley and
Potter in 1976. (2) Most patients with this disease are diagnosed with
AML according to the French-American-British classification (AML-FAB) of
M2 or M4, and many show evidence of underlying or preceding
myelodysplasia. Prognosis is usually poor, and complete remission is
achieved with conventional chemotherapy in only 50% of the cases. Median
survival is dismal (<1 year from diagnosis). Bone marrow
transplantation improves survival and may be the only modality to
achieve cure.

CLINICAL FEATURES

Acute myelogenous leukemia with t(6;9)(p23;q34) usually affects
younger individuals (median age, 23 years; range, 2-66 years), with both
sexes equally affected. Most patients present with de novo AML. The
symptoms are due to the cytopenias often seen in leukemia (fatigue
because of anemia, bleeding tendencies because of thrombocytopenia, and
increased incidence of infections).

LABORATORY FINDINGS

The complete blood cell count in t(6;9) AML usually reveals
findings of bone marrow failure. There is anemia, thrombocytopenia, and
circulating blasts (Figure 1). Children tend to present with lower
hemoglobin values and higher white blood cell counts than adults. In
fact, adults usually present with white blood cell counts lower than
those seen in AML in general. (3)

PATHOLOGY

The bone marrow (Figure 2) in t(6;9) AML is usually hypercellular
for the patient's age, with increased blasts (>20%), which may
show Auer rods or granules. Interestingly, bone marrow basophilia and
unilineage or multilineage dysplasia may be seen in residual
hematopoietic cells (4) (Figures 3 and 4). Alsabeh et al (5) reported an
increased incidence of ringed sideroblasts as a sign of myelodysplasia
in these patients. To our knowledge, the only large retrospective study
(6) to date on this entity (collaboration of 5 cooperative groups:
Southwest Oncology Group, Cancer and Leukemia Group B, Eastern
Cooperative Oncology Group, Children's Oncology Group, and
Children's Cancer Group) reported a prevalence of 44% for marrow
basophilia and 67% for myelodysplasia evidence.

Immunophenotypically, t(6;9) AML blast cells are positive for CD9,
CD13, CD33, and HLA-DR;usually positive for CD45 and CD38;and may be
positive for CD15, CD34, and terminal deoxynucleotidyl transferase. (7)
Alsabeh et al (5) reported an initial immunophenotype of [CD34.sup.-]
blasts, which usually relapsed as [CD34.sup.+].

ANCILLARY STUDIES

Cytogenetic analysis and molecular detection of the DEK-NUP214
fusion gene by polymerase chain reaction (PCR) are very important
ancillary studies in making the diagnosis of AML with t(6;9). This is
demonstrated by a morphology (usually AML-FAB-M2 or occasionally M4) and
immunophenotype that are not diagnostic of this entity. Presence of
basophilia and myelodysplasia in an AML-FAB-M2 suggest this diagnosis,
but it must be confirmed by cytogenetic and molecular analysis.

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In most cases, t(6;9) is the only cytogenetic aberration (Figure
5). It results in a chimeric fusion gene between DEK (6p23) and NUP214
(9q34;formerly known as CAN). This fusion gene encodes a messenger RNA
involved in leukemogenesis. The NUP214 is a putative oncogene with
multiple fusion gene-peptide sequence motifs, and it encodes a nuclear
pore complex protein associated with nucleocytoplasmic transport. It may
be activated by fusion of its 3' end to other genes, like DEK. (8)

The breakpoints in DEK and NUP214 are tightly clustered within
introns, which allow detection of DEKNUP214 fusion transcripts by
molecular techniques like reverse transcriptase-PCR and Southern blot
analysis. Garcon et al (8) studied real-time quantitative PCR levels of
DEK-NUP214 transcripts in 12 patients. They found that molecular
analysis of DEK-NUP214 is useful in follow-up of the disease and that
reverse transcriptase-PCR that is negative for DEK-NUP214 correlates
strongly with patient survival.

Although it may be true that the DEK-NUP214 fusion gene is
responsible for the poor prognosis, studies have demonstrated a high
prevalence (70%) of the /ms-related tyrosine kinase 3-internal tandem
duplication (FLT3-ITD) mutation among patients with t(6;9) AML. (6,9)
That prevalence is definitely higher than that seen in all other types
of AML. The FLT3 gene is involved in the proliferation and
differentiation of hematopoietic stem cells. The mutation is one of the
most frequent mutations in AML and correlates with poor clinical
outcome. (10) Thiede et al (9) showed that patients with t(6;9) and
FLT3-ITD mutations have higher white blood cell counts, higher
percentages of bone marrow blasts, and significantly lower rates of
complete remission.

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CURRENT THERAPY

The type of chemotherapy and autologous stem cell transplant does
not change the poor outcome for patients with AML and t(6;9) and
FLT3-ITD mutations. Allogeneic stem cell transplant may, however, be
associated with better overall survival, similar to all AML with poor
cytogenetic risk factors. t(6;9) AML may warrant a prospective
multicenter investigation of aggressive and novel therapeutic
strategies. A clinical trial of anti-CD33-based therapies plus an FLT3
inhibitor for patients without suitable donors may be helpful.

PROGNOSIS

Patients with AML and t(6;9) have very poor prognoses, short
disease-free survival times, and dismal overall survival rates.6 Age is
not a prognostic factor. High peripheral white blood cell counts
correlate with a poor outcome in overall survival, and high marrow blast
counts are predictive of short disease-free survival times. Patients who
achieve prolonged molecular remission fare better.

CONCLUSION

Patients with t(6;9) AML have a very poor prognosis. The currently
available chemotherapy does not seem to improve overall survival.
However, accurate diagnosis is crucial because these patients may
benefit from early allogeneic stem cell transplant. Improved novel
therapies such as anti-CD33-based therapies and FLT3 inhibitors need to
be evaluated. Molecular follow-up of minimal residual disease is helpful
in stratifying risk and in managing a patient's disease. The World
Health Organization classification of hematopoietic tumors stresses the
importance of prognostic implications of cytogenetic abnormalities in
hematopoietic malignancies. Acute myelogenous leukemia with
t(6;9)(p23;q34), although rare, warrants being added as a separate
category under AML with recurrent cytogenetic abnormalities because of
its distinct morphologic, cytogenetic, and clinical features.